1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to the etching of silicon nitride selective to titanium silicide and the formation of multi-level contact openings for quartermicron devices using an oxide etch followed by an in-situ silicon nitride etch selective to titanium silicide.
2) Description of the Prior Art
In semiconductor manufacturing, such as a DRAM device, silicon nitride hard masks are often used to protect gates during subsequent processing. In these same devices, titanium silicide is often used, such as on source and drain regions. Typically, both the silicon nitride hard mask and the titanium silicide have an overlying dielectric layer composed of one or more layers of silicon oxide, BPTEOS, PETEOS, and the like. It is desirable to form contact openings to form interconnections to the gate and titanium silicide. However, to form these contact openings, it is necessary to etch through the dielectric layer and the silicon nitride hard mask without damaging the titanium silicide. As device geometry is scaled down to quartermicron and below, the aspect ratio increases to greater than 8, further complicating the contact opening etch.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following US Patents.
U.S. Pat. No. 4,529,476 (Kawamoto et al.) teaches a CH2F2xe2x80x94O2 etch through a BPTEOS/PETEOS oxide and a SiN hard mask.
U.S. Pat. No. 5,728,619 (Tsai) discloses a CxHyFz etch through oxide and stopping on a TiSix layer.
U.S. Pat. No. 5,366,590 (Kadomura) teaches an oxide etch using a high density plasma with high selectivity to Si3N4.
U.S. Pat. No. 5,468,342 (Nulty et al.) shows an oxide etch using a SiN hard mask using CH2F2.
U.S. Pat. No. 5,786,276 (Brooks et al.) teaches a CxHyFzxe2x80x94O2 etch of SiN over SiO2.
U.S. Pat. No. 5,258,096 (Sandhu et al.) teaches an etch through for a BPTEOS/PETEOS oxide stopping on a conductive landing pad.
It is an object of the present invention to provide a method of etching silicon nitride selective to titanium silicide.
It is another object of the present invention to form low resistance, multi-level contact openings on DRAM devices without damaging titanium silicide layers that are exposed during the etching process.
It is another object of the present invention to etch through a silicon nitride hard mask in a contact opening having an aspect ratio greater than 8, using an etch selective to titanium silicide.
It is yet another object of the present invention to perform the etches of the previous objects in-situ.
To accomplish the above objectives, the present invention provides a method for etching silicon nitride selective to titanium silicide and a two step etch for fabricating multi-level contact openings on a quartermicron device. The process begins by providing a substrate (10) having thereover a silicon nitride hard mask (20) at one level and a titanium silicide layer (30) at another level wherein the silicon nitride hard mask (20) and the titanium silicide region (30) have an oxide layer (40) thereover. In the first step, the oxide layer (40) is patterned to form a first contact opening (25) and a second contact opening (35). The first etch step is performed using C4F8/CO/Ar/O2 chemistry. The first contact opening (25) stops on the silicon nitride hard mask (20) and the second contact opening (35) stops on the titanium silicide region (30). In the second etch step, any remaining oxide and the silicon nitride hard mask (20) are etched through in the first contact opening (25) using an etch selective to titanium silicide. The etch comprises CH2F2 and O2 at a ratio of CH2F2 to O2 of between about 2 and 4. A key feature of the invention is the second etch step ratio of CH2F2 to O2 which controls the selectivity of nitride to titanium silicide.
The present invention provides considerable improvement over the prior art. A key advantage of the present invention is the second etch step selectivity of silicon nitride to titanium silicide. The inventors have discovered that controlling the O2 flow rate in a CH2F2 silicon nitride etch, has the unexpected result of etching silicon nitride with high selectively to titanium silicide. This etch selectivity enables them to thoroughly etch through a silicon nitride hard mask (20) forming a low resistance first contact opening (25) without etching through a titanium silicide region (30) in the second contact opening (35).
Because the etch selectivity allows the silicon nitride hard mask to be thoroughly etched through, a contact resistance of less than 10 Ohms can be acheived, providing enhanced performance.